1. Field of the Invention
The present invention relates to an apparatus and a method for controlling an electric motor by feedback control to control a motion of a movable object, and also relates to an apparatus and a method for controlling an electric motor by pulse width modulation (PWM).
2. Discussion of Related Art
There is known an apparatus arranged to control an electric motor by feedback control to control a motion of a movable object. This type of motor control apparatus includes an encoder for detecting an actual amount of motion of the movable object driven by the electric motor, and is arranged to control the electric motor such that the detected actual amount of motion coincides with a desired or target value.
In the motor control apparatus described above, the counted number of pulses of an output signal of the encoder is used as a feedback input representative of the actual amount of motion of the movable object. To improve the accuracy of coincidence of the actual amount of motion with the target value, it is required to reduce a minimum amount of motion that can be detected by the encoder, that is, to increase the resolution of detection of the actual motion of the movable object. For example, the resolution of detection can be increased by counting both of rising and falling edges of the encoder signal pulses, or by counting pulses of a plurality of output signals of the encoder which have a predetermined phase difference.
However, an increase in the resolution of detection of the actual motion of the movable object results in an increase in the counted number of the encoder signal pulses for unit amount of the motion, and accordingly an increase in the required memory areas of a register used to count the number of the encoder signal pulses and an arithmetic unit used to perform arithmetic operations for the feedback control of the electric motor.
Where the actual amount of motion of the movable object is detected with a relatively low degree of resolution to reduce the required memory areas of the register and arithmetic unit, an error of detection of the actual amount of motion tends to be large when the target amount of motion is relatively large. Accordingly, the relatively low resolution of detection of the actual amount of motion of the movable object has a risk of failure to accurately control the electric motor for coincidence of the actual amount of motion with the target value.
Therefore, the conventional motor control apparatus is required to detect the actual amount of motion of the movable object with a relatively high degree of resolution, for accurately control the electric motor even when the target amount of motion of the movable object is relatively small. The conventional motor control apparatus is further required to use a register and an arithmetic unit having memory areas large enough to permit the detection of the actual amount of motion with the relatively high resolution when the target amount of motion is relatively large. In this arrangement of the motor control apparatus, the arithmetic operations performed by the arithmetic unit involve a comparatively large number of encoder signal pulses and result in a relatively large load on the apparatus for the feedback control of the electric motor when the target amount of motion (target distance of movement) of the movable object is large. When the target amount of motion is relatively small, on the other hand, the efficiency of utilization of the memory areas of the register and arithmetic unit is extremely low since the counted number of the encoder signal pulses is small.
For example, an electric motor is used to rotate a paper feed roller in a paper feeding mechanism of a printer, for feeding a sheet of paper. The electric motor is driven according to a pulse signal (PWM signal) generated by pulse width modulation (PWM). Namely, an amount of electric current to be applied to the electric motor is controlled by controlling the duty ratio of the PWM signal, so that the operating speed of the electric motor is controlled.
To increase an efficiency of a printing operation performed by the printer, it is required to increase a speed of feeding the paper sheet. To this end, a relatively high voltage is generally applied to the electric motor, and the electric motor is operated at a high speed during an initial portion of an operation of the motor to achieve a feeding motion of the paper sheet, by application of a relatively large amount of electric current to the motor, with the PWM signal being controlled to have a relatively high duty ratio.
To assure a high degree of positioning accuracy of the paper sheet, the duty ratio of the PWM signal is gradually reduced to slowly decelerate the electric motor as the actual distance of movement of the paper sheet approaches a target value. In this respect, it is noted that the paper sheet would have a variation in its stop position, if the electric motor were operated at a high speed immediately before the electric motor is braked at a position at which the actual distance of movement of the paper sheet has almost reached the target value. Namely, the electric motor operating at a high speed cannot be instantaneously brought to a complete stop.
When the target distance of movement of the paper sheet is relatively small, the duty ratio of the PWM signal is controlled to be relatively low even in an initial portion of the operation of the electric motor, so that the electric motor is operated at an accordingly low speed, permitting the paper sheet to be stopped exactly at a position corresponding to the target distance of movement.
When the target distance of movement of the paper sheet is small while the voltage to be applied to the electric motor is relatively high, however, the range in which the duty ratio of the PWM signal is variable so as to feed the paper sheet at a low speed tends to be narrow, and a dynamic range of the electric current applicable to the electric motor is accordingly narrow, making it difficult to intricately control the operating speed of the electric motor.
In the known printer, therefore, the operating speed of the electric motor when the target distance of movement of the paper sheet is relatively small has a relatively large amount of variation, and an accordingly large amount of variation in the stop position of the paper sheet at which a printing operation is effected.